Yersinia pestis is the most highly virulent extracellular pathogen known and poses an ominous threat as an agent of bioterrorism. The bacteria are highly infectious by aerosol and cause pneumonic plague, which can kill in as little as two days after exposure. There is no vaccine available for plague. Vaccines under development for prevention of plague contain two proteins called LcrV and FI. They provide moderate protection against pneumonic plague, but not if the infecting strain lacks the F1 capsular protein. F1 is not required for virulence by the aerogenic route, and its gene is easily deleted, posing the potential of a weaponized Fl-lacking strain against which these candidate vaccines do not protect effectively. To find alternative vaccine candidates, we will develop a novel proteomics approach to identify Y. pestis surface proteins that are expressed during pneumonic plague. These studies will provide the technical base for a more detailed screen for new plague vaccine candidates. Further, we hope that one or more of the proteins we identify in the proposed pilot studies will prove in the future to be protective against pneumonic plague caused by a non-encapsulated strain against which vaccines currently under development will not protect adequately. Our aims are: 1. Develop a biotinylation-MS approach for identification of surface-exposed proteins on Y. pestis recovered from lungs of mice with pneumonic plague. 2. Identify up to 10 surface proteins and evaluate them for conditions that optimize expression.